Apparatus and method of electromagnetically reading coordinate data

Claim 2

In a method of reading coordinate data using an electromagnetic digitizer system including a tablet in which two mutually perpendicular sets of parallel conductors are arrayed so as to substantially define a rectangular Cartesian coordinate system, and a cursor coil which is movable along the surface of said tablet, said method comprising: for each axial direction of said coordinate system, supplying an alternating current to said cursor coil so that plotting the electromotive forces induced in said conductors by said cursor coil against the relative position between said cursor coil and said conductors will give a substantially symmetrical distribution curve with respect to an axis of symmetry passing the center of said cursor ciol, measuring said electromotive forces, and determining the position of said axis of symmetry on the basis of said distribution curve.

Claim 3

The method according to claim 1, wherein said alternating current is supplied sequentially to said conductors, not to said cursor coil, and the electromotive forces correspondingly induced thereby in said cursor coil are measured.

Claim 4

The method according to claim 1, wherein said conductors have one ends thereof connected together so that the respective adjacent conductors form loops and wherein a pair of loops are selected on each side of said axis of symmetry of said distribution curve so that two straight lines connecting the respective two points thereon corresponding to the thus selected respective pairs of loops have an equal gradient in absolute value, and the position of said axis of symmetry is determined as an intersection of said two straight lines.

Claim 5

The method according to claim 1, wherein said conductors have one ends thereof connected together so that the respective adjacent conductors form loops and wherein one loop is selected on each side of said axis of symmetry at such positions equally spaced apart from a loop close to said axis incurring the highest electromotive force, and the position of said axis of symmetry is determined by measuring the differential electromotive force between the thus selected two loops and applying the resultant differential electromotive force to a function experimentally predetermined between the differential electromotive force and said relative position.

Claim 6

The method according to claim 4, wherein said resultant differential electromotive force is compaired with precollected data representing said function.

Claim 7

The method according to claim 1, wherein said conductors have one ends thereof connected together so that the respective adjacent conductors form loops and wherein feedback control is provided between said loops and said cursor coil so that the sum of electromotive forces in two loops, which are selected one on each side of said axis of symmetry at such positions equally spaced apart from a loop incurring the highest electromotive force, is always kept constant.

Claim 8

The method according to claim 1, wherein said conductors have one ends thereof opened or grounded, with said cursor coil and said tablet being adapted so as to produce a double-peak distribution curve symmetrical with respect to said axis of symmetry, and wherein one conductor is selected on each side of said axis of symmetry at such positions equally spaced apart from a conductor close to said axis of symmetry incurring the lowest electromotive force, and the position of said axis of symmetry is determined by measuring the differential electromotive force between the thus selected two loops and applying the resultant differential electromotive force to a function predetermined between the electromotive force and said relative position.

Claim 9

The method according to claim 7, wherein said resultant differential electromotive force is compared with precollected data representing said function.

Claim 10

The method according to claim 7, wherein feedback control is provided between said conductors and said cursor coil so that the sum of electromotive forces in said selected two conductors is always kept constant.

Claim 11

An electromagnetic digitizer apparatus for performing the method according to any of claims 1 to 9, comprising a tablet in which two mutually perpendicular sets of parallel conductors are arrayed so as to substantially define a rectangular Cartesian coordinate system, and a cursor coil which is movable along the surface of said tablet and, for each set of conductors, means for supplying alternating current to said cursor coil or said conductors, means for measuring the electromotive force induced in said conductors or cursor, respectively, depending on the relative position of the center of said cursor coil and said conductors, and means for determining from the electromotive forces induced in or by selected conductors at both sides of the center of said cursor coil the position of said cursor coil, wherein said cursor coil is dimensioned so as to produce a distribution of electromagnetic force which extends over a plurality of conductors and has substantially linear rising and falling portions symmetric to the center of the cursor coil, and wherein said selected conductors are selected to lie on said substantially linear portions.